1. Field of the Invention
The present invention relates to an electromagnet apparatus and a magnetic resonance imaging system (hereinafter referred to as MRI system) using the electromagnet apparatus.
2. Description of the Related Art
An MRI system measures an electromagnetic wave which a hydrogen nuclear spin discharges by a nuclear magnetic resonance phenomenon (hereinafter referred to as NMR), processes the electromagnetic wave as a signal, and thereby makes a tomogram of an inside of a patient body by a hydrogen nucleus density. In order to measure the electromagnetic wave discharged by the hydrogen nuclear spin, it is necessary to generate a homogeneous magnetic field region that has a high strength (not less than 0.2 T) and a high homogeneity of a static magnetic field.
Because a strength of a magnetic field due to an electromagnetic wave discharged by a hydrogen nuclear spin is proportional to that of a static magnetic field in a homogeneous magnetic field region, it is also necessary to increase a strength of the static magnetic field in order to enhance a resolution of a tomogram. Furthermore, in order to make the tomogram have a high image quality and a high resolution and to make a strain thereof none, it is also necessary to heighten a magnetic field homogeneity of the homogeneous magnetic field region. Then as a method of increasing the strength of the static magnetic field of the homogeneous magnetic field region and heightening the magnetic field homogeneity of the region is disclosed an example of using a ferromagnetic member (for example, see Published Japanese translation of PCT international publication for Patent Application Ser. No. 2003-513436 (paragraphs 0018 to 0019, FIG. 3).
The longer a distance from a superconductive coil becomes, the smaller and the more inhomogeneous a magnetic field strength of a static magnetic field generated by the superconductive coil of an electromagnet apparatus becomes; however, it is possible to dispose a ferromagnetic member in the static magnetic field and to heighten the magnetic field homogeneity.
However, it is thought that: considering a magnetic circuit, because a ferromagnetic member lowers a magnetic reluctance, magnetic flux lines tend to further concentrate on the ferromagnetic member placed near a superconductive coil where a magnetic path is short and the magnetic flux lines tend to concentrate before the placement, and thereby the ferromagnetic member becomes a state of a magnetic saturation; whereas the magnetic flux lines become difficult to concentrate on a ferromagnetic member placed away from the superconductive coil, and the ferromagnetic member becomes a state of a magnetic unsaturation.
Consequently, such an electromagnet apparatus is requested that enables also a ferromagnetic member placed away from a superconductive coil to be magnetically saturated, can lessen a magnetization variation of a ferromagnetic member due to a magnetic unsaturation, and can shorten work hours for shimming.